Why is C incorrect?

@zgnewquist, Happy to help. Let's take a look at (C) and (E). (C) says, "Low levels of carbon dioxide in cabin air make it impossible for airborne illnesses to spread." (C)'s wording is far too strong to be supported by the stimulus. We know high levels of carbon dioxide make it easier for airborne illness to spread, but we have no idea if low levels make it impossible for airborne illnesses to spread. We can infer that it's less likely, but we can't infer that it's impossible, so (C) is out. (E) says, "In 1980 the level of carbon dioxide in the cabin air on a two-hour commercial airline flight was lower than it is today on a similar flight." Assuming everything else is equal, the more frequent replenishment taking place prior to 1985 would allow for comparatively lower levels of carbon dioxide than on flights today. We know that prior to 1985, flights replenished air every 30 minutes. 1980 is before 1985, so flights that occurred in 1980 would have replenished air every 30 minutes, so (E) is strongly supported by the stimulus. Does this make sense? Let us know if you have any more questions!

Ok awesome, that is very helpful. Thank you!